The present invention relates to improvements in signal processing systems and more particularly to improved techniques and devices for providing independent samples from a multiple signal environment.
Generally, signal processing systems are designed to separate desired information from unwanted information for evaluation. As is known in the art, in a system such as a radar system, the characteristics of the receiving antenna are such that the desired signal in the main-lobe cannot be distinguished from undesired signals in the side-lobes. This allows an interference (jammer) source to effectively mask the information contained in a radar signal.
Conventionally, side-lobe canceller systems have been used to eliminate the interference by using auxiliary antennas spaced about the radar antenna to sample the interference environment in which the in radar is operating. As taught by patent 3,202,990 to Paul W. Howells, each auxiliary channel signal is utilized by a canceller loop which measures the correlation between its auxiliary signal and the radar output. Each canceller loop then uses the measured correlation to phase shift and attenuate its auxiliary signal, and subtracts the resultant from the radar output at a point ahead of where it measures the correlation. In so doing, the canceller loop attempts to decorrelate the subtractor output from its auxiliary signal and provide a radar output substantially free from interference.
In systems exposed to only one interference source and having one canceller loop, conventional cancellers work well since the only way the correlation can go to zero is for the magnitude of the correlated component out of the subtractor to go to zero. However, when more than one interference source is involved, conventional cancellers cannot work well even though multiple canceller loops are employed. The problem encountered when multiple interference sources are present is that the measured correlation in the canceller loops can go to zero because the vector sum of the correlation of the various jamming signals can go to zero with finite magnitude signals out of the subtractor. This situation prevents accurate cancellation and is considered to be a false zero.
In an attempt to eliminate false zeros and provide more accurate cancellation, the prior art utilized parallel recirculation canceller loops also shown by Howells. Such loops, however, are prone to instability because of the high dynamic ranges and high loop gains required for cancellation.
In copending U.S. Application No. 499,260 entitled "Improved Sidelobe Canceller System" to Bernard L. Lewis and Irwin D. Olin, and 499,374 entitled "Modified Sidelobe Canceller System" to Bernard L. Lewis, filed in the United States on even date herewith and assigned to the same assignee as the present application, iterative systems are disclosed which increase cancellation accuracy and prevent false zeros. The techniques, however, require many iterative loops in some situations, and if partially correlated auxiliary signals are present, are limited in cancellation by common false zeros.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide independent samples, particularly for use in a canceller system.